ext4: fix comments to say "ext4"

Change second/third to fourth.

Signed-off-by: Shen Feng <shen@cn.fujitsu.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/ext4_sb.h b/fs/ext4/ext4_sb.h
index 5802e69..4de9a75 100644
--- a/fs/ext4/ext4_sb.h
+++ b/fs/ext4/ext4_sb.h
@@ -25,7 +25,7 @@
 #include <linux/rbtree.h>
 
 /*
- * third extended-fs super-block data in memory
+ * fourth extended-fs super-block data in memory
  */
 struct ext4_sb_info {
 	unsigned long s_desc_size;	/* Size of a group descriptor in bytes */